undo: reorder cylinders on remove-cylinder undo/redo

The cylinders in the events must be reordered if we remove
a cylinder. To avoid duplication of code, move the reordering
function into qthelper.cpp, though it might not be ideal
there.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/qthelper.cpp

@@ -1661,3 +1661,36 @@ extern "C" char *get_changes_made()
 	else
 		return nullptr;
 }
+
+// Generate a cylinder-renumber map for use when the n-th cylinder
+// of a dive with count cylinders is removed. It fills an int vector
+// with 0..n, -1, n..count-1. Each entry in the vector represents
+// the new id of the cylinder, whereby <0 means that this particular
+// cylinder does not get any new id. This should probably be moved
+// to the C-core, but using std::vector is simply more convenient.
+// The function assumes that n < count!
+std::vector<int> get_cylinder_map_for_remove(int count, int n)
+{
+	// 1) Fill mapping[0]..mapping[n-1] with 0..n-1
+	// 2) Set mapping[n] to -1
+	// 3) Fill mapping[n+1]..mapping[count-1] with n..count-2
+	std::vector<int> mapping(count);
+	std::iota(mapping.begin(), mapping.begin() + n, 0);
+	mapping[n] = -1;
+	std::iota(mapping.begin() + n + 1, mapping.end(), n);
+	return mapping;
+}
+
+// Generate a cylinder-renumber map for use when a cylinder is added
+// before the n-th cylinder. It fills an int vector with
+// with 0..n-1, n+1..count. Each entry in the vector represents
+// the new id of the cylinder. This probably should be moved
+// to the C-core, but using std::vector is simply more convenient.
+// This function assumes that that n <= count!
+std::vector<int> get_cylinder_map_for_add(int count, int n)
+{
+	std::vector<int> mapping(count);
+	std::iota(mapping.begin(), mapping.begin() + n, 0);
+	std::iota(mapping.begin() + n, mapping.end(), n + 1);
+	return mapping;
+}